I had a PCI based 10baseT NIC apparently fail under XP a few days ago and spent several hours troubleshooting the problem before I realized what had happened. XP Pro has a check box in the Power Management tab of the NIC properties box that allows XP to turn the NIC off to save power; it's primarily useful for laptops.
XP turns them off really good, but does not turn them back on when XP is used again. I had to power down, remove all power, not just the XP turn off the computer function, to restore the NIC.
This was a Realtek NIC, not your average expensive NIC, so the problem might be isolated to this type of NIC with XP.
Anyway, if a NIC apparently fails, check to see if XP has powered it down before replacing the NIC and the patch cord.